1. Field of the Invention
The invention relates in general to a programming method and a memory device using the same, and more particularly to a programming method capable of making a threshold voltage more precise, and a memory device using the same.
2. Description of the Related Art
Non-volatile memories are widely applied to various products, such as mobile phones, digital cameras or personal digital assistants (PDAs). The non-volatile memories may be classified into many types, one of which is a flash memory that is frequently seen.
A memory array has many memory cells for trapping charges. The trapped charges influence the state of the threshold voltage of each memory cell so that the object of storing data can be achieved. That is, the stored data content can be obtained according to the state of the threshold voltage of the memory cell. Furthermore, in the memory array having many memory cells, the physical properties of the memory cells are not completely the same. So, the threshold voltages of the memory cells are not completely the same even if the same data is stored in the memory cells. Consequently, the state of the memory array is typically represented by the threshold voltage distribution.
FIG. 1 (Prior Art) is a schematic illustration showing an example of the threshold voltage distribution of a memory array. In this example, it is assumed that each memory cell can store one bit of data, and the state of the memory cell may be one of threshold voltage states S1 and S2. Because the threshold voltage distribution influences the operation of reading the memory cell, a read error tends to occur if the margin of the threshold voltage distribution of the threshold voltage represented by the threshold voltage state becomes wider. Therefore, it is an important subject of the industry to narrow the margin of the threshold voltage distribution of the memory array when the memory cells are programmed.